TLDR: This addon genuinly tries to do what it promises but Firefox defaults + servers = useless addon.
The addon does something, but it's not resulting in refusal of webp images.

By clicking the "W" button on the toolbar, the sent "Accept" headers either include or don't include "image/webp", but it's of no use at all if the default following is "*/*".

It's possible to change the value of the "image.http.accept" key in "about:config" to (for example)
image/png,image/jpeg;q=1
but this addon will then not be able to change this value anymore.

Furthermore, websites such as Youtube (Google in general) just plain refuse to offer anything but webp, even if Firefox does not indicate support for it in the "Accept" header. You may need to fool these websites in assuming you're using an old browser by changing or emptying the "general.useragent.override" key.

Edited at 14/01/2024: changed rating 2»4; addon is not to blame for servers ignoring user preferences, as indicated in response by addon author.

Worked for about 5 minutes and then failed, allowing webp files through. I thought it was great while it lasted. It might be that the web site anticipated the add-in and bypassed it's functionality of not recommending webp images.

This works immediately. Thank you so much!

Thank you so much for this add-on. I could rant all day about how the webp and the gifv formats are a pain in the ass. But I am surprised not a single image application is adding support for these formats. Not counting MS Paint which seems to do webp. How did all these sites suddenly add support for these formats when no editors have? Until I can view these files in my Directory Opus and ACDSEE and can freely convert from one to another I don't need or want it.
Again! Well done on this addon

Does exactly what I wanted. Thanks!

Edit: Well now I'm getting the thrice cursed webp again. Why are companies always pushing for things nobody wants or asked for??

Edit: Doesn't work at all

Very useful, I use it daily on reddit.com

I hate webp. Imagine having to do extra step no one asked about before you can send image to your friend?
Well, this addon works well with reddit

with this extension websites like wikia and such that offer webp in addition to other extensions will finally send the proper file. no more webp pseudo drm to prevent/make it harder for users to download and use normal images.

Using this to download images from Reddit and other websites without that stupid webp format. Why you do this, Firefox?

Use this extension to prevent Firefox from saving New York Times images as webp. Works great, thank you.

It works, thank you.

Thank you i was tired for that file type.
Now google want to monopolize the images...